Carbonate-containing raw materials for the production of calcined or sintered products, e.g. cement clinker, can be subjected to calcination or firing in a rotary kiln, downstream of which is provided a cooler in which the hot cement clinker is contacted by cooling air, the resulting heated air serving elsewhere in the process.
For example, this air can be utilized to preheat the mineral meal, i.e. the crude solid powder, before it is introduced into the rotary kiln.
Fuel is introduced into the system at at least two locations. For example, some fuel may be introduced into the kiln at its solids-outlet end for combustion with some of the preheated air from the cooler, the resulting combustion gases passing from the kiln in counterflow to the solids moving from the inlet end to the outlet end. The major portion of the fuel, however, may be utilized in a precalcination and preheating operation since a substantial portion of the decidification of the raw materials, i.e. removal of carbon dioxide from the carbonates, can be effected in this precalcination stage.
A system of this type is described in my U.S. Pat. No. 4,248,639 and my published European patent document No. 79 100049.0.
Other relevant art includes the publications listed in my U.S. patent and in the German patent document No. 26 29 082 and German patent document No. 25 35 438.
In my European patent document and the aforementioned U.S. patent, I have described a process for the firing of carbonate-containing raw materials for the production of cement clinker, hereinafter referred to as the meal, which comprises a suspension-type preheater for preheating this meal, a precalcinator into which the preheated meal is fed and in which the deacidification reaction is carried out, a rotary kiln receiving the precalcined, deacidified product from the precalcinator and fired to sinter the product to cement clinker, and a cooler receiving the hot clinker and passing the same in direct heat exchange with combustion air.
A portion of this combustion air is used to sustain combustion with a portion of the fuel to fire the rotary kiln and thereby produce hot exhaust gases which travel in counterflow to the solids in the kiln and are discharged through the calcinator which is in the form of an upright shaft at the inlet end of the kiln.
The balance of the combustion air is introduced into this shaft and, in this system, the fuel is sprayed into an exhaust gas/raw meal suspension with the intention of uniformly depositing the fuel particles on the raw meal particles to thereby achieve a direct heat transfer from the fuel to the raw meal particles as the fuel, on these particles, is burned in the heated air.
Experience has shown that, in spite of efforts to obtain as uniform a coating of the fuel on the raw meal particles as possible, concentrations of fuel and locations of reduced fuel density may occur and local heating can result.
This can give rise to localized regions of high temperature levels at which the particles sinter to the walls of the precalcinator or other negative effects on the process.
Such conditions become more pronounced as the time between contact of the fuel with the exhaust gas/raw meal suspension to the beginning of combustion becomes shorter. A gasification of the fuel as is highly desirable for uniform heat transfer, is slowed by the contact of the fuel with the particles and the resulting cooling effect in many cases.